Scientists at the University of California, Berkeley devise a litmus test that changes color in the presence of airborne toxins.

Suppose a scientist could wave a simple, color-coded plastic card that could rapidly determine the chemical composition of the air and whether there were dangerous concentrations of any toxins. Researchers at the University of California, Berkeley, have proposed a similar solution in a paper published in Nature Communications.

The proposed sensor relies on bacteriophage, a benign virus tuned to self-assemble into colorful patterns when exposed to specific toxins. "Our phage litmus is a new type of colorimetric sensor, developed using viral particles, that exhibits different colors upon exposure to different chemicals," says Seung-Wuk Lee, a bioengineering professor at Berkeley and coauthor of the study.

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Phage litmus is the newest biosensor, a device that combines a living component—in this case, a virus—with a physical or chemical detector. As proof of concept, the researchers customized their phage litmus to detect trace amounts of TNT and a host of dangerous volatile chemicals. The team at Berkeley also developed a smartphone app that uses the onboard camera to quickly translate subtle color differences into useful chemical data.

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Turkeys inspired Lee's biosensor. These birds change their skin color in response to stressful situations, thanks to bundles of collagen in their necks. Lee wondered if he could harness the versatility of turkey collagen to create an inexpensive, color-coded biosensor.

"Our virus has a filamentous shape that closely mimics collagen," Lee says. "We utilized the virus as a building block to create a bundled structure similar to the collagen found in turkey skin."

Lee and his team genetically engineered viruses that bind specific sites on TNT and then implanted the phage onto a plastic-like card made of silicone and gold. The current model can detect airborne TNT particles at 300 parts per billion—equivalent to about 1 square inch of grass in a football field. But that is not sensitive enough. "In order to deploy into the field we need much more sensitivity," says Lee. "Down to the one-part-per-trillion level."

It may be a while before Lee's phage litmus can be fine-tuned to detect such low concentrations of TNT. But demand for color-coded sensors abound in the medical field, where Lee imagines a phage litmus could be fine-tuned to detect things such as sugar levels in diabetic patients.